Cellular support for loudspeakers, including acoustic chambers



July 12,1949. G. M. GIANNINI 2,475,732

CELLULAR SUPPORT FOR LOUDSPEAKERS INCLUDING ACOUSTIC CHAMBERS Filed Dec. 3, 1942 2 Sheets-Sheet 1 INVENTOR. GABRIEL M. G/ANN/NI ATTORNEYS y 1949. G. M. GIANNINI v CELLULAR SUPPORT FOR LOUDSPEAKERS,

INCLUDING ACOUSTIC CHAMBERS Filed Dec. 5, 1942 2 Sheets-Sheet 2 Ll. I

2 o r g N (\1 CU N (\1 INVENTOR.

GABRIEL M. GIANNINI ATTORNEYS Patented July 12, 1949 CELLULAR SUPPORT FOR LOUDSPEAKERS, INCLUDING ACOUSTIC CHAMBERS Gabriel M. Giannini, North Hollywood, Calif., as-

signor, by mesne assignments, to Automatic Electric Laboratories, Inc., a corporation of Delaware Application December 3, 1942, Serial No. 467,695

2 Claims.

The present invention relates to sound translating devices and to methods of making the same. This application is a continuation-in-part of the copending application of Gabriel M. Giannini, Serial No. 419,165, filed November 14, 1941, now Patent No. 2,341,539, granted February 15, 1944.

Heretofore it has been proposed to provide a sound translating device comprising an outer shell formed of metal or the like, an electromagnetic unit housed and rigidly supported by the shell in spaced relation with respect thereto, and a layer of fibrous material interposed between the shell and the unit and constituting a cellular filler for the space therebetween, whereby the cellular chamber thus formed effects a desired influence upon the vibrating diaphragm of the unit.

While a sound translating device of this character is reasonably satisfactory in service, it embodies certain objectionable features in that it is extremely diflicult accurately to control the effective or average density of the cellular chamber and its consequent influence upon the diaphragm of the electromagnetic unit. Also the unit is rigidly connected mechanicall to the outer shell and consequentl to the external support which results in the transmission of vibrations and noises between the unit and the external support, whereby the diaphragm of the unit is adversely influenced, causing distortion in the response thereof.

Accordingly, it is an object of the present invention to provide a sound translating device comprising a chamber formed of cellular material housing and resiliently supporting the electromagnetic unit and provided with an outer shell substantially impervious to air, wherein the shell and the unit cooperate to define a cellular chamber housing the casing, the cellular chamber communicating with the [vibratory diaphragm of the unit in order to influence the response thereof.

Another object of the invention is to provide a sound translating device comprising a casing of the general character noted; wherein the casing is formed of a plurality of sections of cellular material of different densities, whereby a correspond in plurality of cellular chambers are provided which communicate with the vibratory diaphragm of the unit in order mutually to influence the response thereof.

Another object of the invention is to provide a sound translating device comprising a cover formed of cellular material, and provided with an outer shell substantially impervious to air, wherein the shell cooperates with the diaphragm of the electromagnetic unit to define a cellular chamber housing the cover, the shell and the cover having communicating openings formed therein providing a sound passage connecting the diaphragm and the outside air, whereby the cellular chamber influences the response of the diaphragm.

A further object of the invention is to provide a hollow self-supporting casing adapted to receive and resiliently to support the electromagnetic unit of a sound translating device, wherein the casing comprises wall structure molded of cellular material and an outer coating providing a shell substantially impervious to air.

A. still further object of the invention is to provide an improved method of making a hollow casing of the character noted for receiving and for resiliently supporting the electromagnetic unit of a sound translating device.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which Figure 1 is a longitudinal sectional view of a sound translating device embodying the present invention; Fig. 2 is an exploded view, partly broken away, of the electromagnetic unit incorporated in the device shown in Fig. 1; Fig. 3 is an enlarged fragmentary sectional view of the upper portion of the unit shown in Figs. 1 and 2; Fig. 4 is an enlarged longitudinal sectional view of a modified form of the diaphragm mounting of the unit shown in Figs. 1 to 3, inclusive; Fig. 5 is a longitudinal sectional view of a cover casing member which may be secured to the open top of the body casing member of the device shown in Fig. 1; and Fig. 6 is a longitudinal sectional view of a modified form of the sound translating device embodying the present invention.

Referring now more particularly to Figs. 1 to 5, inclusive, of the drawings, the sound translating device there shown is of the electrodynamic type and comprises a self-contained electromagnetic unit Hi. The unit ll] includes a permanently magnetized annular ring I00 formed of Alnico or the like. a bottom plate Illl, a cylindrical center pole piece M52 and a top plate I03. Preferably, the parts H51. 885.. and H13 are formed of Allegheny Electric Metal or other high permeable magnetic material. The center pole piece I02 is provided with a lower portion i020. of reduced diameter which extends within a centrally disposed circular opening provided in the bottom plate I I. A sweat connection between the side walls of this opening and the side walls of the lower portion I02a is used rigidly to secure the center pole piece I02 to the bottom plate IOI. At its upper end the center polepiece I02-is provided with a portion I021) of reduced diameter which extends within a centrally disposed circular opening I03a formed in the top plate I03. As best shown in Fig. 1 of the drawings, the diameter of the opening M3311 is somewhat larger than the diameter of the upper portion I02b of the center'pole piece I02; and the upper end of the center pole piece I02 is concentricall disposed within the opening I03a, whereby an annular air gap is formed between the adjacent side walls of the opening I03a and the upper portion I022) of the center pole piece I02. The magnetic circuit of the unit I0 is completed b means of three clamping screws I04 which extend through openings provided in the bottom plate ml and are threaded into tapped wholes I2! provided in the top plate I03. The iscrews I04 serve rigidly to clamp the annular ::permanent magnet I00 between the top and bottom plates I03 and NH.

The moving system of the unit l0 comprises a cone-shaped diaphragm I05 which is formed of pressed fiber and is carried by a supporting structure which comprises a bottom ring I06, six spacing collars IO'Iga pair of clamping rings I08 and "I09, two paper washers H0 and HI and six assembly screws H2. .The character of the diai' phragm I05 depends upon whether the device s to be utilized as a transmitting element or as a receiving element. In the event the device is to be utilized as a transmitting element or transnmitterpthe diaphragm I05 is of one-piece construction and is provided with an outer fiat annular ring portion I05a which extends between wand is cemented to the paper washers IN] and.

III and is uniformly clamped around its pe- :riphery between thewashers H0 and III by the two clamping rings I08 and I09. On the other hand, in the event the device is to be used as 53. receiving element or receiver, the diaphragm constructionillustrated in Fig.4 of the drawings is used. In this construction only the bulbous portion of. the diaphragm I05 is constructed of molded or pressed fiber, the periphery of the diaphragm I05 being secured to an annular ring i I05b formed of treated silk which is disposed between and cemented to the paper washers IIOa and Illa. The purpose of this arrangement i1- :lustra-ted in Fig. 4 of the drawings is to permit substantially free vibration of the diaphragm -l05, particularly at the low frequenciesof the .:operating frequency range, whereby the response -of the-device as a receiver is enhanced at these lfrequencies. On the other hand, when the one- .-piece diaphragm arrangement illustrated in Figs.

-.1 to 3, inclusive, of thedrawings is used, the diaphragm I05 isconsiderably stiffer and, ac- ".cordingly, the response thereof, particularly low tfrequencies, is substantially less than when the diaphragm construction illustrated in Fig. 4 is :used. By virtue of'thisincreased stiffness of the one-piece diaphragm arrangement illustrated in Figs. l to 3, inclusive of the drawings, low 0 frequency noise components of sound are prevented from producing any substantial response of. the device.

.A moving coil-H3 is cemented to the lower bulbous portion of the diaphragm I05 and is concentrically disposed within the air gap defined by the upper portion I02b of.the pole piece I02 net I00 and the center pole .2. string supporting arrangement is provided. 'This arrangement comprises three connected strings H m, I Nb and I40 which extend through equiangularly spaced openings cut through the lower portion of the diaphragm I05 and are joined together, as indicated at IMd, within the dished cavity of the diaphragm I05. The free ends of the three strings are respectively anchored by clamping them to the lugs II5a, I151), not shown, and H50 which extend radially inward and are turned upward from the supporting ring Small clamping plates IISa, IISb, not shown, and H00 disposed between shortened spacing collars I 07 are used to clamp the free ends of the springs lI4a, II4b and H40 against the lugs Il5a, H513 and 50. Preferably, the three strings H la, 4b and H40 are formed of silk fishline or the like and it will be understood that by appropriately tensioning the three strings the required support for the moving coil I I3 may be obtained.

In order to seal the annular cavity I28 formed between the spaced-apart annular permanent magnet H00 and the center pole piece I02 from the acoustical cavity III formed adjacent the lower side of the diaphragm I05, a sealing ring IIS is provided which snugly encloses the upper end portion I021) of the center pole piece I02 and is clamped to the under side of the top plate I03 by means of assembly screws II9. Preferably, the sealing ring H8 is formed of rubber, molded Bakelite or other insulating material having nonmagnetic properties. This sealing arrangement, including the sealing ring I I0, prevents the first-mentioned annular cavity, formed between the annular permanent magnet I00 and the center pole piece I02, from communicating with the acoustical cavity II'I provided adjacent the lower side of the diaphragm I05, thereby to reduce the effective volume Of the acoustical cavity l I? and consequently to enhance the stillness thereof, whereby the damping action exerted upon the diaphragm I05 is enhanced.

In the assembly of the unit I0 the magnetic circuit structure is built by first securing the sealing ring I I8 to the under side of the top plate I03 and then setting the assembly screws I04 to clamp the annular permanent magnet I00 between the top and bottom plates I03 and MI.

After the magnetic structure is thus assembled the portion of the sealing ring H 8 facing toward "the air gap of the structure may be painted with shellac or other sealing compound to provide an acoustical seal which blocks off the annular cavity, formed between the annular permanent magpiece I02, from the acoustical cavity II'I adjacent the under side of the diaphragm I05. The diaphragm assembly is prepared by cementing the two paper washers I H) and HI to the top and'bottom surfaces of the fiat annular portion I 05a of the diaphragm I05, assuming that a transmitter is being constructed, and by cementing the moving coil II3 to the lower'central portion of the diaphragm I05.

If the device is to beused as a receiver, the diaphragm structure illustrated in Fig. 4 of the drawings is constructed by first cementing the paper washers II and III to the opposite peripheral surfaces of the silk ring I051), after which the inner upper edge surfaces of the silk rin I051) are cemented to the under side of the diaphragm I05. Following the assembly of the diaphragm structure the three strings II Ia, H41) and I I40 are drawn through the angularly spaced openings provided in the base of the diaphragm I05. After this operation is performed the supporting ring I045, the clamping rings I08 and I09 and the assembled diaphragm structure may be stacked on the top plate I03 and the assembly screws I I2 may successively be inserted through the registering openings provided in the stacked clamping rings and diaphragm structure, the spacing collars I01 and the supporting ring I06. Initially the screws which extend through the clamping elements Ilfia, H01) and H60 are not tightened but the alternate assembly screws which extend only through alternate spacing collars I0'I are tightened sufiiciently to provide a semirigid assembly. Following this operation, the ends of the strings IIIa, HM) and H40 may be threaded between the clamping elements I Ito, IIBb and H60 and their associated lugs IIa, I I52) and H50 and properly tensioned to provide the required later-a1 axial support for the diaphragm I05.

I I2 may be tightened securely to clam the ends of the strings II Ia, H41) and II I-c between the clamping elements IIBa, H01) and I I50 and their respective associated lugs IIEa, H51) and H50. Thus the structure of the electromagnetic unit I0 is completed.

Also the sound translating device comprises a body casing member II having the form of a hollow block of acoustical insulating material I20; the electromagnetic unit I0 being arranged and snugly received within the opening formed in the hollow block I20. Preferably, the hollow block I20 is formed by a molding process described more fully hereinafter and consists principally of a fibrous insulating material and a suitable binder; whereby the molded acoustical block I20 is of cellular construction and is somewhat porous so that air can be transmitted therethrough. The outer surfaces of the molded block I20 including the bottom wall and the connected side wall are provided with one or more coats of lacquer or paint I25 in order to minimize or substantially to exclude the transmission of sound therethrough. In the illustrated embodiment shown in Fig. 1 of the drawings, the molded block I20 may be either round or square in transverse cross-section and is provided with a cylindrical opening therein which snugly receives the side walls of the electromagnetic unit I0, as previously noted. In the event the sound translating device is to operate as a nondirectional device, the assembly is completed by closing the open top of the hollow block I20 with a silk screen I23 having approximately the characteristic impedance of air over which is placed a thin sheet of aluminum I2 l having a number of holes of appropriate size cut therethrough for the purpose of transmitting sound to or from the diaphragm I05 of the enclosed electromagnetic unit I0. A clamping ring I29 and assembly screws I3t may be used to hold the aluminum plate I24 and the sill: screen I23 in assembled relation on the open top of the insulating block I20, the assembly screws being arranged to pass through registering openings Thereafter the assembly screws 6 provided in the clamping block, the aluminum plate I24 and the molded block I20.

In the event directional characteristics are to be imparted to the sound translating device, a cover casing member I3 is provided, as illustrated in 5 of the drawings, which comprises a block of acoustical insulating material I2 I. Preferably, the block. I2I is formed by a molding process described more fully hereinafter and consists principally of a fibrous insulating material and a suitable binder; whereby the molded block I2I is of cellular construction and is somewhat porous so that air can be transmitted therethrough. The outer surfaces of the molded block I2I including the top wall and the connected side wall are provid'ed with one or more coats of lacquer or paint I26 in order to minimize or substantially to exclude the transmission of sound therethrough. In the illustrated embodiment shown in Fig. 5 of the drawings, the molded block I2I may be either round or square in transverse cross-section in order to match the transverse crosssection of the molded block I20 illustrated in Fig. 1. The molded block I2I is provided with a number of sound transmitting passages I22 molded or out there through along the axis of sound transmission to and from the diaphragm I0 5 of the associated electromagnetic unit I0. These passages are of the correct dimensions, that is, diameter and length, to provide for the desired sound energy transfer between the atmosphere and the acoustical cavity disposed adjacent the outer surface of the diaphragm I05. The molded block I2I may be secured to the molded block I20 to clamp the aluminum sheet I24 and the silk screen I23 to the open top of the molded block I20 by means of assembly screws I3I passed through registering openings in the two molded blocks I20 and IZI.

In view of the foregoing description of the construction and arrangement of the electromagnetic unit I0 and the body casing member II of the sound translating device, it will be understood that a first acoustical chamber III is provided between the under side of the diaphragm Ill-5 and the upper surface of the top plate I03; a second acoustical chamber I I'm is provided between the upper side of the diaphragm I05 and the under surface of the aluminum plate IN; and a third acoustical chamber is formed by the wall siruc ture of the molded block I20 itself. The third acoustical chamber formed by the wall structure of the molded block I20 itself is defined between the outer side walls of the annular permanent magnet Hill, the outer bottom wall of the bottom plate I0 I, the under surface of the aluminum plate I26 and the outside coating of lacquer I25. Accordingly, the third acoustical chamber is of cellular construction, being entirely filled with the fibrous material of which the block I20 is molded.

The third acoustical chamber has a total volume determined by the exterior dimensions of the molded block I20 and the exterior dimensions of the electromagnetic unit Ill, that is, the wall space of the molded block I20 between the exterior surface of the unit I0 and the interior surface of the coating of lacquer I25. However, the effective volume of the third acoustical chamber is determined in accordance with the density of the fibrous material of which the block I20 is molded, which in turn determines the porosity of the cellular structure thereof. Thus, it will be understood that, by varying the dimensions of the molded block I20 and consequently the total volume of the third acoustical chamber and also by varying the density of the fibrous material of which the block I20 is'molded and consequently the effective volume of the third acoustical chamber, the characteristic of the third acoustical chamber may be determined. More specifically, the variation of the two factors mentioned above establishes the efiective volume of the third acoustical chamber and consequently the mass and stiffness of the air entrapped in the cellular structure thereof, as well as the damping characteristic of the third acoustical chamber upon the diaphragm I05.

Finally it is noted in this connection that the third ascoustical chamber of cellular construction communicates both with the first and second acoustical chambers, whereby the first and second acoustical chambers respectively disposed below and above the diaphragm I are interconnected by a controlled passage; the controlled passage constituting the fibrous or cellular wall structure of the molded block I extending below and above the peripheral edge of the diaphragm I05. Accordingly, the character of the third acoustical chamber determines the amount of damping which is exerted upon the passage of air between the first and second chambers through the wall structure of the molded block I20 incident to the vibration of the diaphragm I05.

Also it will be understood that the sound translating device comprises a fourth acoustical chamber which is formed by the wall structure of the cover casing member I3, whereby the sound transmitting passages I22 formed in the molded block I2I communicate both with the second acoustical chamber and the fourth acoustical chamber. More particularly, the fourth acoustical chamber is formed by the wall structure of the molded block I'2I itself, as noted above, and is defined between the upper surface of the aluminum plate I24 and the outside coating of lacquer I26. Accordingly, the fourth acoustical chamber is of cellular construction, being entirely filled with the fibrous material of which the block I2I is molded.

The fourth acoustical chamber has a total volume determined by the exterior dimensions of the molded block I2I and the dimensions of the sound transmitting passages I22 formed therein. However, the effective volume of the fourth acoustical chamber is determined in accordance with the density of the fibrous material of which the block I2I is molded, which in turn determines the porosity of the cellular structure thereof. Thus it will be understood that, by varying the dimensions of the molded block I2I and consequently the total volume of the fourth acoustical chamber and also by varying the density of the fibrous material of which the block I2I is molded and consequently the effective volume of the fourth acoustical chamber, the characteristic of the fourth acoustical chamber may be determined. More specifically, the variation of the two factors mentioned above establishes the effective volume of the fourth acoustical chamber and consequently the mass and stiffness of the air entrapped in the cellular structure thereof, as well as the damping characteristic of the flcairth acoustical chamber upon the diaphragm In the design of the sound translating device it will be understood that the third acoustical chamber may exhibit a considerable damping action upon the diaphragm I05 in the band of frequencies, including the resonant or natural frequency of the diaphragm- I05, and may also exhibit a considerable resonance action at a higher frequency above the resonant ornatural frequency of the diaphragm I05. Similarly, the fourth acoustical chamber may exhibit a slight damping action upon the diaphrag 5 in the band of frequencies, including the resonant or natural frequency of the diaphragm I05, and may also exhibit a considerable resonance action at a still higher frequency above the resonance of the third acoustical chamber. This design of the sound translating device with particular reference to the characteristics of the third and fourth acoustical chambers is productive of a considerably extended and very flat response characteristic,

Referring now more particularly to Fig. 6 of the drawings, a modified form of the sound translating device is shown which is of the electrodynamic type and comprises a self-contained electromagnetic unit 20. Preferably, the unit 20 is identical to the unit I0 previously described. Also this sound translating device comprises a diaphragm 205 which embodies the same general construction as that previously described in connection with Fig. 3 if the device is to be used as a transmitter, and which embodies the same general construction as that previously described in connection with Fig. 4 if the device is to be used as a receiver. More particularly, the diaphragm assembly comprises an annular ring 206 formed of aluminum or the like to which the diaphragm 205 is suitably secured. Also the device comprises a body casing member 2I including a lower cup-shaped element 22 and two ring-shaped elements 23 and 20 arranged in stacked relation, the ring-shaped element 23 being positioned upon the rim of the cup-shaped element 22 and the ring-shaped element 24 being positioned upon the ring-shaped element 23. The body casing member 2I has the form of a hollow block of acoustical insulating material; the electromagnetic unit 20 being arranged and snugly received within the opening formed in the hollow block 2!. Preferably, the hollow block 2I is formed by a molding process described more fully hereinafter and consists principally of a fibrous insulating material and a suitable binder, whereby the molded acoustical block 2I is of cellular construction and is somewhat porous so that air can be transmitted therethrough. Also this device comprises a cover casing member 25 in the form of a block of acoustical insulatin material and provided with a number of sound transmitting passages 20'I molded or cut therethrough along the axis of sound transmission to and from the diaphragm 205 of the unit 20. Preferably, the block 25 is formed by a molding process described more fully hereinafter and consists principally of a fibrous insulating material and a suitable binder; whereby the molded block 25 is of cellular construction and. is somewhat porous so that air can be transmitted therethrough,

The annular ring 206 carrying the diaphragm 205 is positioned on the upper surface of the ring-shaped element 24 and the block 25 is positioned upon the annular ring 206. Also a sheet of paper or the like 208 impregnated with lacquer or paint is interposed between the cup-shaped element 22 and the ring-shaped element 23; and a sheet of paper or the like 209 impregnated with lacquer or paint in interposed between the two ring-shaped members 23 and 24. The body casing member 2! and the cover casing member 25 are retained in assembled relation by a number of assembly screws 2| 0 extending through aligned openings provided therein, the outer ends of the assembly s'c'rev'vs 210 being threaded and carrying suitable nuts 2| l. The outer surfaces of the molded block 2| including the bottom wall and the connected side wall, as well as the outer surfaces of the molded block 25 including the top wall and the connected side wall, are provided with one or more coats of lacquer or paint M2 in order to minimize or substantially to exclude the transmission of sound therethrough. The molded blocks 2| and 25 may be either round or square in transverse cross-section in the illustrated embodiment shown in Fig. 6. The passages 20l provided in the block 25 are of the correct dimensions, that is diameter and length, to provide for the desired sound energy transfer between the atmosphere and the acoustical cavity disposed adjacent the outer surface of the diaphragm 205.

In view of the foregoing description of the construction and arrangement of the electromagnetic unit 20 and the body casing member 2| of the sound translating device, it will be understood that a first acoustical chamber 2l3 is provided between the under side of the diaphragm 205 and the upper surface of the associated top plate 203; a second acoustical chamber 2M is provided between the upper side of the diaphragm 295 and the under surface of the molded block 25; a third acoustical chamber is formed by the wall structure of the ring-shaped element 2d itself; and a fourth acoustical chamber is formed by the wall structure of the ring-shaped element 23 itself. The third acoustical chamber formed by the Wall structure of the molded ringshaped element 24 itself is defined between the under side of the annular ring 206, the upper side of the impregnated sheet 209 and the outside coating of lacquer 212; while the fourth acoustical chamber formed by the Wall structure of the molded ring-shaped element 23 itself is defined between the under side of the impregnated sheet 299, the upper side of the impregnated sheet 28 and the outside coating of lacquer 2l2. Accordingly, the third and fourth acoustical chambers are of cellular construction, being entirely filled with the fibrous material of which the respective ring-shaped elements 24 and 23 are molded.

The third acoustical chamber has a total volume determined by the exterior dimensions of the molded ring-shaped element 24; however, the effective volume of the third acoustical chamber is determined in accordance with the density of the fibrous material of which the ring-shaped element 2a is molded, which in turn determines the porosity of the cellular structure. Thus it will be understood that, by varying the dimensions of the molded ring-shaped element 24 and consequently the total volume of the third acoustical chamber and also by varying the density of the fibrous material of which the ring-shaped element 24 is molded and consequently the effective volume of the third acoustical chamber, the characteristic of the third acoustical chamber may be determined. More specifically, the variation of the two factors mentioned above establishes the effective volume of the third acoustical chamber and consequently the mass and stiffness of the air entrapped in the cellular structure thereof, as wellas the damping characteristic of the third acoustical chamber upon the diaphragm 205.

The fourth acoustical chamber has a total volume determined by the exterior dimensions of the molded ring-shaped element23; however, the effective volume of the fourth acoustical chamber is. determined in accordance with the density of 10 the fibrous material of which the ring-shaped element 23 is molded, which in turn determines the porosity of the cellular structure thereof. Thus it will be understood that, by varying the dimensions of the ring-shaped element 23 and consequently the total volume of the fourth acoustical chamber and also by varying the density of the fibrous material of which the ring-shaped element 23 is molded and consequently the effective volume of the fourth acoustical chamber, the characteristic of the fourth acoustical chamber may be determined. More specifically, the variation of the two factors mentioned above establishes the effective volume of the fourth acoustical chamber and consequently the mass and sti1f-.

ness of the air entrapped in the cellular structure thereof, as well as the damping characteristic of the fourth acoustical chamber upon the diaphragm 205.

Further it is noted in this connection that the first acoustical chamber 2l3 communicates with both the third and fourth acoustical chambers of cellular construction, whereby the third and fourth acoustical chambers commonly communicating with the first acoustical chamber 213 are interconnected by a controlled passage; the controlled passage including the fibrous or cellular wall structure of the molded ring-shaped elements 2 and 23. Accordingly, the characters of the third and fourth acoustical chambers determine the amount of damping which is exerted upon the passage of air from the first acoustical chamber 2l3 through the wall structures of the molded ring-shaped elements 24 and 23 incident to the vibration of the diaphragm 205. Preferably, the ring-shaped elements 23 and 24 are formed of fibrous material of different densities, whereby the third and fourth acoustical chambers exhibit different amounts of damping upon the passage of air to and from the first acoustical chamber 2i3 incident to the vibration of the diaphragm 205.

Also it will be understood that the sound translating device comprises a fifth acoustical chamber which is formed by the wall structure of the cover casing member .25, whereby the sound transmitting passages 20! formed in the molded block 25 communicate both with the second acoustical chamber 2 I l and the fifth acoustical chamber. More particularly, the fifth acoustical chamber is formed by the wall structure of the molded block 25 itself, as noted above, and is defined between the upper surface of the annular ring 26 and the outside coating of lacquer 252. Accordingly, the fifth acoustical chamber is of cellular construction, being entirely filled with a fibrous material of which the block 25 is molded.

The fifth acoustical chamber has a total volume determined by the exterior dimensions of the molded block 25 and the dimensions of the sound transmitting passages 21 formed therein. However, the effective volume of the fifth acoustical chamber is determined in accordance with the density of the fibrous material of which the block 25 is determined, which in turn determines the porosity of the cellular structure thereof. Thus it will be understood that, by varying the dimensions of the molded block 25 and consequently the total volume of the fifth acoustical chamber and also by varying the density of fibrous material of which the block 25 is molded and consequently the effective volume of the fifth acoustical chamber, the characteristic of the fifth acoustical chamber may be determined. More specifically, the variation of the two factors menthe mass and stiffness of the.=air. entrapped in the cellular structure thereof, aswell as the damping characteristic. of the fifth acoustical chamber upon the diaphragm:205.- Finally it is pointed out that the. sound translatingsdevice comprises a further sealed chamber'which-is formed by the wall structure of the cup-shaped element 212. The last-mentioned sealed chamberformed by the Wall structure of thekmolded cup-shaped element 22 itself is defined between the outer walls of the electromagnetic unit 20, the under surface of the impregnated -sheet 208 and the outside coating of lacquer i2l 2. The last-mentioned chamber issubstantially sealed from the fourth-acoustical chamber-of cellular construction formed -by the molded-ring-shaped element 23 due to the imposition of the impregnated sheet 208 between the molded cup-shaped element 22 and the molded ring-shaped element 23. and the snug fit of the electromagnetic unit20 in the opening provided in the body casing member 2 I.

In the designof the sound translating device, it will Ice-understood that-thethird and fourth acoustical chambers may-exhibit different and considerable damping actions upon the dia-- phragm 205 111 the band of -frequencies, including the resonant or natural frequency of the diaphragm l2ll5, and may also exhibit different and considerable resonance actions at two adjacent higher. frequencies-above the resonant or natural. frequency of-the diaphragm 295. Similarly, the fifth acoustical chamber'may exhibit a slight damping action uponthe diaphragm 205 in the band of frequencies, including the resonant ornatural frequency ofthe diaphragm 205, and

may alsoexhibit considerable resonance action the blocks 120' and I21 in the first form of the soundtranslating device :andthe blocks 2| and 25 in the second form of the sound translating device'maybe molded A mold-of the required configurationis first selected, having given volumetric dimensions. The fibrous material utilized is of the rock-wool type and may comprise the material of which the air-acoustic sheets sold commercially under the tradename J ohns-Manville are made. This roughfibrous material is thoroughly mixed with a suitable working agent, suchas acetone, and a suitable binder, such as the cementsold under the trade name Ducco cement, in order to form a fairly smooth mixture in the general form of a' plaster; A batch of the prepared plaster is then poured into the mold and allowed to set; Wherebythe acetone evaporates and the Ducco cement binds the fibrous material into a-block having a cellular construction and being rather porous'having a rather low density. After the block has set, .it is removed from the mold and theoutside'surfaces thereof are covered or. painted with a suitable lacquer in order to provide an outer shell which'greatly minimizes or substantially 'preventsthe' passage of air and sound therethroughto the atmosphere. The thickness of the'coating of lacquer on the outer surface of the block is determined in accordance 40*?new cover casing member are-thenassembled with with 'the de'gree' to Whichlit is desirable to ex clude the passage of'l'airand sound through the block to the "atmosphere: .In' the event a rather thick coating-of i-lacquer-oni the outer surface of the block is'idesired, it maybeiobtainedby first wrapping the outer surface with-a thinlayer of adhesive :or masking tape and then spraying the tape with a heavy coatingof lacquer.

In producing a sound :translating device, a' 'body casing:member and a cover casing mem ber, in the event the latter-casing member is re-' quired, are 'suitablyfmolded in the manner explained aboveg and an electromagnetic unit-l0 of the requiredlcharacter is' assembled in the man- .ner previously explained; The unit is then assembled'in the bodyicasingmember and thecover casing member is then assembled thereon in the event it isrequireda The assembly is then completed by suitably securing togetherthe unit, the: body' casing 'member and the cover casing member, The completedfl assembly is then mounted upon a support utilizing-clamps or the like'engaging the body casing member and anchored to the: supportm Then-the assembled and gjmounted sound translating device is tested in body. casing member and in the cover casing member are made-i For example, a new body' casing member and a new cover casing member may be molded, havingsmaller orlarger dimensions' and molded offibrous'material of the same aidensity. Oh the other hand, a new body casing member and-"a new cover casing-member may be moldedphaving the same dimensions but comprising fibrous' material of lesser or greater density; The new body casingmember and the respect to the electromagnetic unit, and the sound translating device-is-again-tested in order to determine the-operatingcharacteristic thereof. The above-described methodis repeated until a sound translatingdevice'having the required operating characteristics is produced.

In placing in service a sound translating device having the:desired'-operatin'g characteristics and produced-by the method described above, preferably, a supporting clamponthe' like is utilized which engages thebody casing mem-ber of the device and is anchored to the 'exte'rn'al support.

This arrangement positively prevents the trans-- mission of sound as well as vibration between the sound translating device and'the' support in view of. the fact that the body-'casing 'member of the device: is rather resilient and constitutes a sound and vibration 'insulator between the electromagnetic unitof thedeviee and the external support.

Hence," this arrangement insulates the diaphragm of the electromagnetic unitof the device from un-' desirablesounds'which would otherwise be transmitted theretofromthe external support through -the supporting structure.

In view of the foregoing considerations it is apparent that thereare provided an improved sound translating device, an improved method of making. a sound translatingdevice, and an im-'- proved process of molding an acoustical casing for a sound "translating device;

While there have' been disclosed what are at present considered to'be the-preferred embodi- 13 are within the true spirit and scope of the invention.

What is claimed is:

1. A sound translating device comprising an electromagnetic unit including a vibratory diaphragm and structure cooperating with said diaphragm to define an acoustic cavity between said diaphragm and said cooperating structure, a hollow casing formed of a plurality of sections of cellular material of different densities engagingly surrounding and resiliently supporting said structure, and an outer shell substantially impervious to air surrounding said casing and coopcrating with said structure to define a plurality of cellular cavities between said outer shell and said structure respectively housing the sections of said casing, said cellular cavities communicating with said acoustic cavity, whereby said cavities mutually influence the response of said diaphragm.

2. A sound translating device comprising an electromagnetic unit including a vibratory diaphragm and structure cooperating with said diaphragm to define an acoustic cavity between said diaphragm and said cooperating structure, a hollow self-supporting casing molded in a plurality of sections of cellular material of different densities, said casing engagingly surrounding and resiliently supporting said structure, and an outer coating on said casing, said coating providing an outer shell substantially impervious to air and cooperating with said structure to define a plurality of cellular cavities between said outer shell and said structure respectively housing the sections of said casing, said cellular cavities communicating with said acoustic cavity, whereby said cavities mutually influence the response of said diaphragm.

GABRIEL M. GIANNINI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 20,155 Hanna Nov. 3, 1936 1,943,258 Harshberger Jan. 9, 1934 2,013,695 Nicolson Sept. 10, 1935 2,031,500 Olney Feb. 18, 1936 2,041,777 Olney et a1 May 26, 1936 2,082,261 Bunyan June 1, 1937 2,293,914 Nanfeldt Aug. 25, 1942 2,315,896 Dumas Apr. 6, 1943 FOREIGN PATENTS Number Country Date 17,984 Australia Jan. 22, 1929 

